[Python-ideas] Fast sum() for non-numbers

[Ron Adam]

On 07/11/2013 09:31 AM, Oscar Benjamin wrote:
> On 11 July 2013 14:20, Ron Adam <ron3200 at gmail.com> wrote:
>>
>> On 07/11/2013 06:05 AM, Oscar Benjamin wrote:
>>> On 11 July 2013 11:47, Ron Adam<ron3200 at gmail.com> wrote:
>>
>>     chain(iters)        # better in most cases...
>
> I think you mean chain.from_iterable rather than chain
>
>> I'd like to see chain as a builtin in any case.
>
> chain.from_iterable should be the builtin not chain.

I agree.


>> How do you feel about adding the ability of sum to sum vectors or lists of
>> values to each other?
>>
>> sum([[x1, y1], [x2, y2], ...])  --->  [x1+x2, y1+y2]
>
> That's the beauty of it. sum() already sums anything you want as long
> as __add__ is implemented. If I wanted to do the above with some
> vectors I would probably use numpy arrays which have precisely the
> __add__ method you want:
>
>>>> import numpy as np
>>>> arrays = [
> ...   np.array([1, 2, 3]),
> ...   np.array([2, 3, 4]),
> ...   np.array([3, 4, 5]),
> ... ]
>>>> arrays
> [array([1, 2, 3]), array([2, 3, 4]), array([3, 4, 5])]
>>>> sum(arrays)
> array([ 6,  9, 12])
>
> This is possible because of the simplicity of the core algorithm in
> sum() i.e. just calling 'total = total + item' in a loop. Anyone who
> wants to use sum() with their own type can already do so. Earlier you
> seemed to be advocating changing that by restricting the types that
> sum() accepts.

I think this is what it should do.

I tried overiding lists __add__, but that didn't work as nice.  It needs to 
have a starting list with all zeros in it.  How does numpy get around that?

Ron